The present invention relates to improvements in or relating to electrochromic systems.
As the energy performance of buildings and automobiles becomes an increasingly important design feature, strategies for optimising performance in this respect are receiving considerable attention.
An important aspect of the energy performance of the above relates to the incident radiation transmitted by the window area of a building. Such concerns are further complicated by the need to ensure occupant comfort. It is in this context that the electrochromic (EC) window technology has assumed increasing importance, the amount of incident radiation transmitted by such windows being electronically controllable. Effective implementation of EC window technology in buildings is expected to provide the following benefits:
1. Reduce adverse cooling effects. Reduce cooling energy. Down-size air conditioning plant. Reduce peak electricity demand.
2. Increase beneficial effects of daylight. Reduce lighting energy. Reduce peak electricity demand.
3. Increase occupant comfort. Increase thermal comfort. Increase visual comfort.
Even greater benefits would be expected to accrue in an automobile, where the ratio of glazed surface to enclosed volume is significantly larger than in a typical building. Specifically, effective implementation of EC window technology in automobiles is expected to provide the following benefits in addition to those in the built environment:
1. Increased motoring safety. Reduced glare. Mirror control. Head-up display.
EC technology is not limited to the applications described above. Others include privacy glass, angle-independent high-contrast large-area displays, glare-guards in electronic devices, electronic scratch pads.
Existing EC devices, including those commercially available, are non-optimal for the large glazing areas encountered in building and automotive applications and are based on technologies which are process and energy intensive. Therefore new EC technologies, resulting in improved device specification and which may be manufactured more easily at a lower cost, will be commercially important. It is noted in this context, that the current market for EC window technologies in buildings and automobiles is estimated world-wide at over $2 billion.
For an overview of these and related topics see the review Large-Area Chromogenics:Materials and Devices for Transmittance Control (Eds. Lampert and Granqvist), SPIE Institutes for Advanced Optical Technologies Series Vol. 4. Existing EC devices are found in one of the two categories outlined below. Firstly, there are those devices based on ion insertion reactions at metal oxide electrodes. To ensure the desired change in transmittance the required number of ions must be intercalated in the bulk electrode to compensate the accumulated charge. However, use of optically flat metal oxide layers requires bulk intercalation of ions as the surface area in contact with electrolyte is not significantly larger than the geometric area. As a consequence the switching times of such a device are typically of the order of tens of seconds.
Secondly, there are those devices based on a transparent conducting substrate coated with a polymer to which is bound a redox chromophore. On applying a sufficiently negative potential there is a transmittance change due to formation of the reduced form of the redox chromophore. To ensure the desired change in transmittance a sufficiently thick polymer layer is required, the latter implying the absence of an intimate contact between the transparent conducting substrate and a significant fraction of the redox chromophores in the polymer film.
As a consequence the switching times of such a device are, as above, typically of the order of tens of seconds.
It is an object of the present invention to provide an improved EC system.
According to the invention there is provided a nanoporous-nanocrystalline film comprising a semiconducting metallic oxide having a redox chromophore adsorbed thereto.
A xe2x80x9cnanocrystalline filmxe2x80x9d is constituted from fused nanometer-scale crystallites. In a xe2x80x9cnanoporous-nanocrystallinexe2x80x9d film the morphology of the fused nanocrystallites is such that it is porous on the nanometer-scale. Such films, which may hereinafter be referred to as nanostructured films, typically possess a surface roughness of about 1000 assuming a thickness of about 10 xcexcm.
The nanostructured films used in the present invention colour on application of a potential sufficiently negative to accumulate electrons in the available trap and conduction band states. As a consequence of the high surface roughness of these films, ions are readily adsorbed/intercalated at the oxide surface permitting efficient charge compensation and rapid switching, i.e. the need for bulk intercalation is eliminated. However, despite the rapid switching times in such films, the associated change in transmittance is not sufficient for a commercial device. To overcome this limitation a redox chromophore is adsorbed at the surface of the transparent nanostructured film which, when reduced, increases the extinction coefficient of an accumulated trapped or conduction band electron by more than an order of magnitude. Further, due to the nanoporous structure and associated surface roughness of the nanocrystalline films used, the redox chromophore is effectively stacked as in a polymer film, while at the same time maintaining the intimate contact with the metal oxide substrate necessary to ensure rapid switching times.
The redox chromophore may be any suitable redox chromophore and preferably comprises a compound of the general formula I 
wherein X is a charge balancing ion such as a ion such as a halide; R1 is any one of the following: 
and
R2 is any one of the following: 
xe2x80x83wherein R1 is as defined above, R3 is any of the formulae (a) to (f) given above under R2, m is an integer of from 1 to 6, preferably 1 or 2 and n is an integer of from 1 to 10, conveniently 1 to 5.
A particularly preferred redox chromophore is a compound of formula II, viz. bis-(2-phosphonoethyl)-4,4xe2x80x2-bipyridinium dichloride 
The semiconducting metallic oxide may be an oxide of any suitable metal, such as, for example, titanium, zirconium, hafnium, chromium, molybdenum, tungsten, vanadium, niobium, tantalum, silver, zinc, strontium, iron (Fe2+ or Fe3+) or nickel or a perovskite thereof. TiO2, WO3, MoO3, ZnO and SnO2 are particularly preferred.
The invention also provides an electrochromic system comprising: a first electrode disposed on a transparent or translucent substrate; a second electrode; an electrolyte; an electron donor; and an electrochromic layer comprising a nanoporous-nanocrystalline film according to the invention intermediate the first and second electrodes.
The substrate is suitably formed from a glass or a plastics material. Glass coated with a conducting layer of fluorine doped tin oxide or indium tin oxide is conveniently used in the EC system of the present invention.
The electrolyte is preferably in liquid form and preferably comprises at least one electrochemically inert salt optionally in molten form in solution in a solvent. Examples of suitable salts include hexafluorophosphate, bis-trifluoromethanesulfonate, bis-trifluoromethylsulfonylamidure, tetraalkylammonium, dialkyl-1,3-imidazolium and lithium perchlorate. Examples of suitable molten salts include trifluoromethanesulfonate, 1-ethyl, 3-methyl imidazolium bis-trifluoromethylsulfonylamidure and 1-propyldimethyl imidazolium bis-trifluoromethylsulfonylamidure. Lithium perchlorate is particularly preferred.
The solvent may be any suitable solvent and is preferably selected from acetonitrile, butyronitrile, glutaronitrile, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methyloxazolidinone, dimethyl-tetrahydropyrimidinone, xcex3-butyrolactone and mixtures thereof.
The electron donor is preferably a metallocene or a derivative thereof. The electron donor is preferably soluble in the electrolyte solvent. Ferrocene is particularly preferred.